- now keep the bit with 0 width to merge them instead of just delete them
 - merging algorithm re-coded (extensively)
 - change behavior of thick polyline to only 1 width value per point instead of 2 per pair of adjacent points.
 - do not post-process the voronoi diagram in polylines, now it's don in expolygon.medial_axis.
 - failsafes in perimeter_generator (too many splits, too small)
 - some new tests.

* corrections from review
* more functions for a clearer code
* now simplify the frontier with the anchor to avoid weird edge-cases.
* new post-process: remove "curve edge" before merging
* new post-process: cube corner: the small bit that go from the voronoi corner to the cube corner is now a "pulling string" that pull the voronoi corner a bit to make a nicer cube.
* _variable_width : reduce the threshold for creating a new extrusion by half vs threshold to create segments (if not, it doesn't create enough)

…wall

add to cmakelist
add needed function (mostly copy from slic3rPE)

…nd perimeter)

…same orientation'

to 'all medial axis segments of a semicircumference have the same orientation (but the 2 end points)'

correct a bug with offset parameters.

some bugfix on the algo of thin_walls.

…_width (can be toggled when calling expolygon.medial_axis).

…wall

 - reworked thin_variable_width (discretize into segments of constant width)
 - bugfix taper_ends
 - add setting thin_walls_overlap to control the perimeter/thin wall overlap
 - add point.interpolate(%,point) : point method

 * concatenate_polylines_with_crossing: now connected the good end with the other good end
 * discretize_variable_width: now use TP.width->flow.spacing for gap-fill and TP.width->flow.width if possible for thin-walls.
 * bugfix thin wall (medial axis): use correct temp variables for main_fusion.

…wall

 - taper_ends fix
 - empty_vector.back() bugfix
 - more accurate min width from nozzle diameter & layer height
 - add epsilon offset when trying to see if points are inside a polygon
 - failsafe not safe (do not fusion with a too high width).
 - add remove_collinear_points() in the simplify function.

adding comments

…wall

add cp medial axis tests